Add --template command line option

The --template (-t) option allows the user to specify
a template for their input other than the single default
template used for pages and posts. This is useful for
cases where many posts with similar contents such as
package release announcements.

Change-Id: Id0ab6dea70983cd8f22235858515db91119ca728
Add unit tests for --template option

Add a few unit tests to verify the default template is selected
correctly if none is specified.

Add mock as a test-time dependency.

Update tox configuration to let user pass arguments to nose, for example
to only run some of the tests.

Change-Id: I231df0051d024393659f5d619f136a0dde1dd1e3
Merge branch 'content-templates' of https://github.com/dhellmann/tinkerer into dhellmann-content-templates

Conflicts:
	tox.ini